Conventionally, magnetic resonance (MR) signals are excited and received by an RF resonator comprised of an RF coil resonating at Larmor frequency. MR experiments with different nuclei or at different static field strengths usually require different RF coils which resonate at a desired frequency or frequencies. Since the discovery of the MR phenomena, resonant RF coils have generally been used for MR signal excitation and reception. Functioned as a filter, the resonance feature of the RF coils help reduce the noise level by rejecting other non MR signals, thereby potentially improving MR signal-to-noise ratio. However, the resonance requirement of RF coils creates many experimental complexities and technical challenges particularly in designing high and ultrahigh field (3T and above) coils, double or multiple-tuned coils and parallel imaging arrays. These technical difficulties/challenges dramatically hinder the development and the use of MR imaging due to the low RF coil quality, significantly reducing efficiency of the MR signal excitation and reception. One example is that in designing large-sized RF coils for high field MR imaging it is extremely difficult (or impossible) to achieve the required high resonant frequency due to the increased self-inductance and parasitic capacitance of the RF coils. Another example is that in designing RF coil arrays for parallel imaging, a daunting technical challenge is to attain sufficient electromagnetic decoupling among the resonant elements in coil arrays. Insufficient decoupling results in low efficiency of RF coil arrays, ultimately leading to low parallel imaging performance.
There is a trend to move to high and ultra high filed applications such as 300 MHz, 400 MHz, and above. With existing methods, it is extremely difficult to design RF coils for high field human MR imaging at high resonant frequencies due to the increased self-inductance and parasitic capacitance present with conventional RF coils. Coils for such applications generally are large and thus difficult to design and build to achieve the required high operating frequency. With conventional systems, the coils are specific to a narrow range of resonant frequencies. Therefore, the coil may need to be changed for different specimens.